For a long time various industries have used, for water treatment, cultures of microorganisms such as bacteria or others which, dispersed in an aqueous medium more or less rich in ionized or non-ionized inorganic elements, are nourished by an organic or inorganic substrate, under well defined operating conditions (aeration or not, temperature, pH, etc.) to perform a certain number of transformations by assimilation and metabolism. For example, according to a now standard water treatment process, microorganisms are used to transform pollutant substances in systems either with activated sludge, in fermentation tanks or ponds or in the form of bacterial beds where the microorganisms are held on a fixed support acting as a filtration layer for the waters to be purified.
In this process of the "fixed bacterial bed" type, various supports can be used that are able to work in an aerobic or anaerobic medium. For example, hydrated silicates such as natural or artificial pozzolanas, or activated carbon, are used. According to a notable improvement, there is specified the use, as the support for holding the microorganisms and for filtration, fired clays (or chamottes) in the form of aggregates containing small amounts of trace elements such as metals of the types: iron, copper, zinc, manganese, cobalt, aluminum, sodium, potassium, calcium, magnesium. These trace elements serve as nutrients for microorganisms and thus it is possible to obtain excellent activation of the biomass whose rate of growth is optimal (French Pat. No. 76.03573 of Feb. 10, 1976).
These fired clays, doped with trace elements, are well suited for biological filtering in an aerobic medium; they actually achieve a spectacular improvement of the nitrification yield (transformation of nitrogen in NO.sub.3 by zoogloea or similar type autotrophic bacteria) in comparison with previously known biological filtering processes.